In many wireless systems, such as a system defined by the proposed 802.11n standard, data packets include preambles, or headers, that precede the data in a frame. Current IEEE 802.11a and IEEE 802.11g protocols utilize such a frame structure.
FIG. 1 shows a legacy data frame (L-Data) structure 10. The L-Data structure 10 includes preamble fields, which are also known as headers, such as a legacy short training field (L-STF) 11, a legacy long training field (L-LTF) 12, and a legacy signal field (L-SIG) 13. Each of the preamble fields in the L-Data frame structure 10 serve a specialized function within the header. For example, the L-STF field 11 is utilized for automatic gain control (AGC), while the L-LTF field 12 may be used for channel estimation and fine time/frequency offset estimation. In addition, the L-Data frame structure 10 includes a legacy data field (L-DATA) 14, which contains the transmitted data. The L-Data frame structure 10 is particularly suited to low throughput of Single-In Single-Out (SISO) data transmission. For a 20 MHz presentation protocol data unit (PPDU), the L-STF 11 and L-LTF 12 fields may be 8 microseconds in length, while the L-SIG 13 field is 4 microseconds in length.
As in nearly any communication system, however, a wireless communication system is subject to noise and interference which can distort the signal and corrupt the reception of the transmitted data. Accordingly, methods and devices for dealing with noise and interference have been employed. Some of these methods, or techniques, attempt to estimate the noise in order to eliminate it from the received signal.
One common technique for noise estimation is the maximum likelihood method. However, a drawback to this method is that the channel state information is required. Another alternative is to use a frequency domain autocorrelation function. This method is troublesome, though, if there is significant frequency variation.
It would therefore be advantageous if a method and apparatus existed that overcomes the drawbacks of prior art wireless systems.